1. Field of Invention
This invention relates to three-dimensional (3D) autostereoscopic displays and other similar electro-optical devices.
2. Introduction
Several 3D displays, such as 3D displays described in 1, 2, 4, 7, 5, 6, and some other similar 3D displays, all suffer from similar problems, such as crosstalk, aberrations, deformed Modulation Transfer Function, etc. As it is obvious from the examples listed here, many 3D displays suffer from crosstalk.
It is possible to try to minimize that crosstalk, or any other distortion in the image, in several ways. The 3D display described in 1 and in 4 is based on the concept of grating cells (FIGS. 1(A), 1(B)). Each pixel of the display is divided into an array of subpixels, and each subpixel is covered with a diffraction grating cell of such period and orientation that it directs light to the center of corresponding (virtual) viewing slit when activated.
The problem with the architecture like the one described in 1, and in 4, is that it requires very small grating cells, and that existing technology cannot provide that. Even if it can provide that, in the design of a 3D display that was described in 1, and in 4, there was a conflict of interests because on one hand smaller grating cells provide more views, etc., while on the other hand smaller grating cells create more crosstalk (since the size of a pixel is limited, and since the sum of the areas of all grating cells is equal to the area of the pixel (so larger number of grating cells results in smaller grating cells), and since the decrease of the size of grating cell results in the increase of crosstalk), and are more difficult to manufacture and are more expensive, etc.
The area of each grating cell, which is a rectangular aperture, causes appearance of side-lobes in spite of sine grating being used for the grating cell, since the entire transfer function of the 3D display analyzed here includes the transfer function of the rectangular aperture as one of its factors. (One another conflict of interest between parametric variables in this design solution is a consequence of the requirement that the grating cell has to be much larger than grating period, and grating period has to be much larger than wavelength of light (in that case sine phase grating functions as a pure ideal steering element), while grating cell has to be as small as possible, as explained here, since the (optimal) size of the pixel is limited, etc. It is also difficult to fabricate grating cells smaller than what was achieved in 4.)
It has to be also emphasized that this Invention, described in this Patent Application, addresses more than just how to decrease the crosstalk in some three-dimensional displays. The quality of the images increases as the size of grating cells, that are actually the samples of Holographic Optical Element (HOE) or of Diffractive Optical Element (DOE), decreases, because the transfer function of HOE in spatial frequency domain becomes closer to ideal as a consequence (since samples of HOE should be infinitely small in ideal case in order that the sampling does not distort the sampled function), and converges to ideal transfer function as the size of grating cells (samples of HOE) decrease. But decrease of the size of samples of the HOE increases crosstalk and/or superposition of extraneous images in the image reconstruction. Therefore, the optimization described in this Patent Application addresses both the need to increase the number of different views of a three-dimensional image (by making it possible to use smaller grating cells in a display), and the need to improve the image quality, as explained above, by making it possible to decrease the size of samples of the HOE/DOE (“grating cells”), with less problems with the crosstalk as a consequence of that. This is important problem in Computer Generated Holography, in the case of pixelated Spatial Light Modulators in general, and in the case of any spatially sampled HOEs/DOEs in general. In addition, the problem with the solutions that are grouping pixels for the purpose of steering the light from them as from the whole groups of pixels to different directions is that the size of the whole group of pixels has to be small enough in order the pixel grouping not to be observable, which makes design synthesis more difficult (for example, that limits the maximum possible size of pixels below the smallest observable size). One other important advantage of this type of display is that there is a possibility to display all views at once, or at least more than one view (multiple views) at once (at least with solutions based on non-coherent illumination), which is very beneficial since it is difficult to achieve very large space-bandwidth per time unit for 3D displays that is needed.
Due to problems with aberrations, large data rates, nonlinearities, blur, need for wide fields of view and for large displays, etc., some kind of projection is very desirable as a design methodology of 3D displays, and one possible implementation of such design solution of a 3D display is a multi-projector and/or a display that can direct light to different directions, like these design solutions of 3D displays mentioned in this section.